Nanofibrillar cellulose (NFC) has recently found applications in various areas, including biomedical and pharmaceutical applications. In higher plants, cellulose is organized in morphologically complex structure consisting of β(1→4) D-glucopyranose chains. These chains are laterally bound by hydrogen bonds to form microfibrils with a diameter in nanoscale, which are further organized in microfibril bundles. Furthermore, cellulose molecules are associated with other polysaccharides (hemicelluloses) and lignin in plant cell walls, resulting in even more complex morphologies. The cellulose nanoscale fibers can be released from the highly ordered structure by mechanical process, combined with other treatments such as enzymatic pretreatment.
Nanofibrillar cellulose is typically obtained by mechanical disintegration of cellulose pulp, carried out with suitable disintegration equipment. Mechanical disintegration is an energy consuming operation where the production capacity is limited. Thus several measures have been proposed for improving the grinding or fibrillation process, such as modification of pulp prior to the disintegration. Said modification may comprise chemical modification of the pulp to yield anionically or cationically charged grades of nanofibrillar cellulose (NFC). Said chemical modification may be based for example on carboxymethylation, oxidation, esterification, or etherification of cellulose molecules. However, said chemical modification methods may result in grades of NFC, which are not desirable for all applications and thus also alternative methods have been studied, such as enzymatic treatment.
U.S. Pat. No. 7,838,666 discloses a fine fibrous water-dispersible cellulose derived from a plant cell wall having starting cellulosic substance, wherein the starting cellulosic substance has an α-cellulose content of 60-90% by weight and an average degree of polymerization of 400-1300, or the starting cellulosic substance has an α-cellulose content of 60-100% by weight and an average degree of polymerization greater than 1300, the water-dispersible cellulose being crystalline having a crystallinity of 55% or more, and fine fibrous without entanglement between fibers, and the water-dispersible cellulose having substantially no branched bundles of fiber, the water-dispersible cellulose comprising 30% by weight or more of a component stably suspensible in water, wherein the component comprises a fibrous cellulose having a length of 0.5-30 μm and a width of 2-600 nm, and a length/width ratio of 20-400, and the water-dispersible cellulose having a loss tangent <1, when made into a 0.5% by weight aqueous dispersion.
Bhattacharya et al. 2012 disclose nanofibrillar cellulose, which contains fiber bundles with a thickness of larger than 50 nm. Although the cellulose nanofibers are very thin their organization into thick bundles results in scattering of light. Light scattering causes limitations of use of NFC hydrogel in applications requiring optical detection e.g. with light microscopy.
Pääkkö et al. 2007 disclose a method of producing cellulose fibrils using a combination of enzymatic hydrolysis and mechanical shearing. They report that previous attempts to prepare MFC only by extensive mechanical shearing resulted in that the homogenizer became blocked and the resulting material was non-homogenous. However, the enzymatic treatment leaves traces of enzymes in the end product and an additional enzyme removal or inactivation step may be required before downstream applications. Additionally, the enzymes have a significant effect on the morphology of the cellulose nanofibrils: enzymatical pre-treament leads to decreased degree of polymerization, decreased length and decreased networking of the cellulose nanofibrils, and may lead to rod-shaped cellulose crystals or whiskers.
Accordingly, there exists a need to provide improved nanofibrillar cellulose and methods for the manufacture of nanofibrillar cellulose.